PROJECT SUMMARY/ABSTRACT Streptococcus pneumoniae is one of the most important bacterial causes of pneumonia, a leading cause of death in young children globally. Widespread use of pneumococcal conjugate vaccines (PCVs) has substan- tially reduced colonization and disease caused by serotypes included in PCVs. With these vaccine successes, efforts are underway to employ reduced-dose PCV schedules in many regions. However, breakthrough infec- tions caused by vaccine serotypes continue to occur, and infections by non-vaccine serotypes are rising. Alt- hough these changes have been largely documented using traditional cultures, new molecular serotyping methods are revealing previously unrecognized colonization complexities, including frequent co-colonization with more than one pneumococcal serotype. Yet to date, application of this approach in longitudinal surveil- lance has been limited. Before reducing PCV doses and reducing their biological pressure, it is critical to un- derstand whether PCV serotypes may be just suppressed, but not fully eliminated. Existing PCVs were de- signed to target the most virulent pneumococcal serotypes, so unveiling potentially unrecognized reservoirs of vaccine serotypes containing virulence genes is critical to informing initiatives to prevent their influence in less pathogenic, but now more prevalent, serotypes. Additionally, clarifying how PCV vaccination impacts the inter- action of PCV serotypes with other now prevalent pneumococcal strains, as well as other NP microbiota, will help clarify their role in pathogenesis of ARI and pneumococcal diseases. To address these critical research priorities, Dr. Howard proposes three integrated Specific Aims: 1) Determine the contribution of PCV7 sero- types in NP pneumococcal colonization following vaccination, 2) Define the determinants of NP pneumococcal density during ARI, and 3) Define the contribution of NP Streptococcus-dominated microbiota profiles to risk of ARI. The RESPIRA-Peru study was designed to assess the epidemiology of respiratory viruses and bacteria, especially S. pneumoniae, and their impact on ARI in children <3 years of age in rural Peru. 892 children en- rolled in the study contributed detailed clinical data and respiratory samples that will serve as the data source for the proposed studies. Aims 1 and 2 will characterize patterns of colonization with single or multiple pneu- mococcal serotypes in children over time, before and after vaccination with PCV7, and the association of these colonization profiles with respiratory disease. Aim 3 will evaluate whether specific microbiota profiles are asso- ciated with higher risk of ARI. The goal of this project is to leverage the resources of Dr. Howard?s outstanding research environment, her expert team of mentors, and her established broad skill set so that she emerges from this award as an independent investigator leading a large research program performing work that clarifies the impact of PCVs on pneumococcal colonization and disease to inform optimized PCV dosing schedules and the development of improved pneumococcal vaccines.